1. Field of the Invention
The invention relates to means for providing mechanical energy to propel vessels and the like. The invention also relates to a method of propelling vessels by mechanical means.
2. Description of the Prior Art
The most conventional and widely used type of marine propeller is the screw type. The propeller is arranged under the hull of the vessel, usually aft, and it is rotably driven by a shaft slanting downwards from the fore end to the aft end where it passes through the hull and is connected to the shaft of the device providing the motor power. The blades of the screw are arranged so as to ensure that the water taken in from in front of them will be thrust backwards at an increased speed. This increase corresponds exactly to the factor by which the required propulsion force is generated.
The apparent simplicity of such a propeller conceals a number of difficulties inherent therein. These difficulties include not only those connected with the process of producing the propeller, such as the necessity of extending the shaft through the hull without causing any leaks in the latter, fixing the bearings which guide the rotation of the shaft, ensuring resistance to vibrations, and the like, but also problems of a technical and even scientific nature. For example, the exact quantitative theory governing the operation of the propeller still remains to be defined and the empirical method is the only way to determine the optimum shape of the propeller blades. It is only by lengthy and laborious adjustments and improvements and by successive tests at sea that the speed of the vessel can be correctly coordinated with the power supplied by the motor, given that at a low speed the thrust imparted to the vessel will be comparatively limited. The impedance or resistance of the marine propeller cannot be regulated, except in the very rare and complex case of a screw mechanically manufactured such that its thread may be varied. Furthermore, the ratio of the power used for the propulsion of the vessel to that actually supplied by the motor remains low, despite almost a century of research in an attempt to improve it. To this day the ratio remains on the order of 0.5. Finally, the screw propulsion system further suffers from the drawback that the propeller is underneath the vessel and thus in a position where damage can be a serious problem due to the difficulty of repairing it.
In order to overcome most of the aforementioned drawbacks, a "hydrojet" or "jet propeller" device based on a different principle has been designed and is now being developed to a considerable extent. The so-called "jet propeller" or "hydrojet" enables the propeller to be dispensed with and comprises a water intake, either of the scoop type or of the parietal type, conveying the water through a conduit, known as a suction intake, to a pump which increases the level of kinetic energy of the water supplied and conveys it aft via an ejection circuit terminating in a nozzle through which the water is finally ejected aft at a high speed.
Such devices represent a definite advance over the prior art and while they make it possible to navigate on shoals and eliminate the need for screw propeller movements, they suffer from numerous drawbacks and particularly from various problems involved in installing the complex and expensive pump equipment.
Although various pump embodiments such as those of the centrifugal or axial type, a pump with elements in parallel or with elements in series, or with a combination of fixed and movable blades, and the like may be used, performance remains subject to the limits imposed by the hydrodynamic principles governing continuous and incompressible fluids. Pumps of this kind, which also suffer from the phenomenon known as cavitation, do not provide an overall propulsion efficiency significantly greater than conventional screw type propellors.